Prior to the events of "The Day of the Doctor", it was implied at least once on screen that Susan - being a Time Lord (or at least a Gallifreyan) - was lost. In "The Empty Child"/"the Doctor Dances", Dr. Constantine said, "When this war [WWII] began I was a father and a grandfather, now I'm neither -- but I'm still a doctor," and the Ninth Doctor replied, "I know the feeling." Though it should be noted the Doctor implied his family was no more as early as "The Tomb of the Cybermen".
Although there was some speculation that Susan might have been forgotten - perhaps to protect her - "The Rings of Akhaten" has the Doctor directly reference Susan, and a photograph of his granddaughter is seen in Day of the Doctor.
The outcome of "The Day of the Doctor", of course, is that Gallifrey was not destroyed after all, but due to shifting timelines, the Ninth Doctor believed the planet and his people to be lost, hence his statement quoted above. Whether Susan actually survived as a result - or if she stayed on earth in the 22nd century as per "The Dalek Invasion of Earth", we do not know. In the BBC Books novels, it is stated that Susan eventually left earth, however.